The goal of this research is to understand the physiological basis of mammalian maternal behavior. The proposed research is focused on a model of mother-young interactions which implicates maternal temperature as the factor limiting the duration of contact between mothers and their young. The research constitutes the first analysis of the physiological basis for moment-to-moment changes in mammalian maternal behavior. The proposed experiments test the specific predictions of the model by continuously recording both maternal physiology and behavior. The first part of the proposal concerns the acute elevation in maternal temperature that limits maternal contact bouts. The role of the acute elevation in maternal brain temperature in limiting contact bout duration under different natural variations in maternal heat load will be determined. The role of REM sleep in altering the vulnerability of mothers to acute hyperthermia during pup contact will then be investigated. The second part of the proposal concerns the role that hormones play in chronically elevating the vulnerability of dams to the acute hyperthermia that limits contact bout duration. The model proposes that progesterone and corticosterone work in concert to elevate the chronic heat load of mothers, thereby making them vulnerable to acute hyperthermia during contact bouts. The specific proposal that progesterone elevates the body temperature that mother rats defend will be evaluated. The role of corticosterone in elevating maternal heat load will then be determined.